We have used the IRAM 30-m and FCRAO 14-m telescopes to observe the molecular clumps
associated with 12 ultracompact (UC) Hii regions in the , 8–7 and
13–12 rotational transitions of methyl-acetylene (CH3C2H). Under the
assumption of LTE and optically thin emission, we have derived
temperature estimates ranging from 30 to 56 K.
We estimate that the clumps have diameters of 0.2–1.6 pc,
H2 densities of 105–, and masses of
102–. We compare these values with those obtained by
other authors from different molecular tracers and find that the
H2 density and the temperature inside the clumps vary respectively like
and , with R distance
from the centre.
We also find that the virial masses of the clumps are
~3 times less than those derived from the CH3C2H column densities: we
show that a plausible explanation is that magnetic fields play an important
role to stabilise the clumps, which
are on the verge of gravitational collapse.
Finally, we show that the CH3C2H line width increases for decreasing
distance from the clump centre: this effect is consistent with infall
in the inner regions of the clumps. We conclude that the clumps around UC Hii
regions are likely to be transient (~105 yr) entities, remnants of
isothermal spheres currently undergoing gravitational collapse: the high mass
accretion rates (~ yr-1) lead to massive star
formation at the centre of such clumps.

Key words: stars: formation / radio lines: ISM / ISM: molecules

*

Based on observations carried out with the IRAM Pico Veleta telescope.
IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

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